This invention relates to a power supply apparatus for a vehicle such as an automobile. More particularly, it relates to a power supply apparatus for a vehicle which can supply power at two different voltages.
A typical automotive vehicle is equipped with a power supply apparatus including a battery and an alternating current generator (referred to as an alternator for short) which can provide electrical power for electric loads in the vehicle and also charge the battery. Most commonly, the battery voltage is approximately 12 volts. A voltage of this level is satisfactory when the electrical load is relatively small. However, in recent years, the electrical load of new automobiles has been steadily increasing. For example, the demand by consumers for increased comfort has resulted in the addition of devices such as electric heaters for rapid defrosting of windshields or heaters for rapid heating of the passenger compartments. Furthermore, the need to reduce exhaust emissions has resulted in the addition of electrical devices such as electric heaters for catalysts. All of these devices contribute to an increase in the overall electrical load.
When the vehicle power supply generates a low voltage of approximately 12 volts, the large electrical load causes the current to become large, resulting in resistance damage to wiring and connectors and making it difficult to control the electric loads.
Accordingly, vehicle power supplies capable of generating two different voltages have been proposed. In these power supplies, a high voltage is generated for loads for which a high supply voltage is advantageous, and a relatively low voltage, such as 12 volts, is generated for other loads, such as electrical lights for which a relatively low voltage is desirable in order to maximize the life span of the filaments.
For example, Japanese Published Unexamined Patent Application No. 55-53152 discloses a power supply arrangement for a vehicle including a first electrical generator with an output voltage of approximately 12 volts and a second electrical generator with a higher output voltage. However, this arrangement has the drawback of requiring two generators, and in modern vehicles, the engine compartments are so crowded that it is difficult to find room to install an additional generator. It has the further drawback that the second generator increases the load on the engine and thereby increases the fuel consumption of the vehicle.
Japanese Published Unexamined Patent Application No. 63-69500 discloses a power supply apparatus in which the alternating current output from a three-phase alternator is rectified to direct current and supplied to low-voltage loads, or else the alternating current output is increased in voltage by a three-phase transformer and then rectified to a high voltage direct current for supply to high-voltage loads. However, this apparatus has the disadvantages of requiring a three-phase transformer and a rectifier for use with the transformer, so the apparatus becomes expensive, and it becomes difficult to guarantee space for the apparatus within the engine compartment of a vehicle.